The current society, also referred to as an information society, has proceeded, accompanied by the discovery of an inorganic semiconductor, which typically includes Si, and by the development of a wide range of the electronic devices using the same. However, the preparation of an electronic device using an inorganic material requires a high-temperature or vacuum process, thereby requiring lots of investments on the equipments, and further has undesirable physical properties for the flexible displays which have recently attracted a great deal of attention as a next-generation display.
In order to solve the above-described problems, there are suggested organic semiconductor materials, which have recently attracted a great deal of attention as a semiconductor material having various physical properties. The organic semiconductor materials can be employed in a variety of the electronic devices, which had previously used inorganic semiconductor materials. Typical examples of the electronic devices using the organic semiconductor materials include an organic light emitting device, an organic solar cell, and an organic thin film transistor.
The organic electronic device, such as an organic solar cell, an organic light emitting device, or an organic transistor, is an electronic device employing the semiconductor properties of the organic semiconductor material, and usually comprises at least two electrodes and an organic material layer between the two electrodes. For example, a solar cell generates electricity using the electrons and the holes, separated from excitons which are generated in an organic material layer by means of solar energy. The organic light emitting device introduces the electrons and the holes from two electrodes to the organic material layer, thereby converting current to visible light. The organic transistor transports the holes or the electrons which are formed on an organic material layer by means of the voltage applied on a gate between a source electrode and a drain electrode. In order to improve the electronic devices, the electronic devices can further comprise an electron/hole injecting layer, an electron/hole extracting layer, or an electron/hole transporting layer.
The organic semiconductor materials which are used for the electronic devices should have good hole or electron mobility. To meet this requirement, most of the organic semiconductor materials are configured to have conjugated structures.
Further, the organic semiconductor materials used in each of the electronic devices have various preferable morphologies, depending on the characteristics required from the devices.
For example, when a thin film is formed using an organic semiconductor material, the thin film in the organic light emitting device preferably has amorphous property, whereas the thin film in the organic thin film transistor preferably has crystallinity.
That is, if an organic thin film in the organic light emitting device preferably has a crystalline property, light emitting efficiency may be reduced, the quenching sites in the charge transport are increased, or the leakage current is increased, thus leading to deterioration of the device performance.
On the other hand, it is favorable that the organic transistor has larger charge transfer property of the organic material layer, and accordingly, it is preferable that packing between the molecules of the organic materials are effective, thereby providing the organic thin film with crystallinity. Most preferably, this crystalline organic film particularly has a single crystal form, and in the case where this crystalline organic film has a polycrystalline form, preferably, the size of each domain is large, and the domains are well connected with each other.
In order to satisfy these requirements, the substances having non-planar structures such as NPB (4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl), and Alq3 (aluminum tris(8-hydroxyquinoline)) are usually used so as to form amorphous thin films in the organic light emitting devices, and the substances having rod-like structures such as pentacene and polythiophene, or the substances having planar structures such as phthalocyanine derivative are usually used so as to easily effect packing between the molecules in the organic transistors.
On the other hand, the organic electronic device comprising at least two organic material layers can be prepared, in which at least two kinds of the organic semiconductor material having different functionalities are laminated, in order to improve the device performance.
For example, the organic light emitting device can further comprise a hole injecting layer, a hole transporting layer, an electron transporting layer, and an electron injecting layer to facilitate injection and transportation of the holes or electrons from the anode or the cathode, thus enhancing the device performance.
In the case of the organic thin film transistor, there is a method, which involves introducing an auxiliary electrode comprising an organic semiconductor, or subjecting the electrode to an SAM (Self Assembled Monolayer) treatment with an organic material to reduce the contact resistance between the semiconductor layer and the electrode. Further, there is used a method, which involves treating the surface of the insulating layer with an organic material to improve the characteristics concerning the contact with a semiconductor.
In addition, the organic semiconductor material used in the organic electronic device preferably has thermal stability against Joule heat which is generated during the transfer of the charges in the device, and also preferably has a suitable band gap, and an HOMO or LUMO energy level for easy injection or transport of the charges. Further, the organic semiconductor material should be excellent in chemical stability, and the electrode should be excellent in the interface characteristics with an adjacent layer, as well as in stability against moisture or oxygen.